Contrasting effects of yeast and bacteria on floral nectar traits

Flowers can be highly variable in nectar volume and chemical composition, even within the same plant, but the causes of this variation are not fully understood. One potential cause is nectar-colonizing bacteria and yeasts, but experimental tests isolating their effects on wildflowers are largely lacking. Vannette and Fukami examine the effects of dominant species of yeasts and bacteria on plants’ reproductive organs with some help from the birds and the bees.

Mimulus aurantiacus, the sticky monkeyflower. Photo by Vannette and Fukami.

The authors conducted this study using micro-organisms isolated from the nectar of flowering plants at Jasper Ridge Biological Preserve (JRBP) in the foothills of the Santa Cruz mountains near Stanford, California. They isolated yeasts and bacteria from the nectar of the flowering shrubs Mimulus aurantiacus (sticky monkeyflower) and Eriodictyon californicum (yerba santa). The plants are both visited by hummingbirds and bees, though M. aurantiacus tends to be pollinated by birds, while E. californicum is pollinated more by insects.

Previous work by Vannette and Fukami has shown that microbes were responsible for a lot of the variation in sugars found in M. aurantiacus, and that there’s not a lot diversity in the microbes found in flowers. So they chose to concentrate on Metschnikowia reukaufii, a yeast with a reputation for altering pollination in plants.

The authors inoculated flowers of M. aurantiacus with yeasts, bacteria, or sugar, and measured flower traits. Although both microbial species grew well in nectar, yeasts decreased amino acid concentration, while bacteria tended to increase amino acid concentration, but reduced sugar concentration and nectar volume.

The results were quite varied. Vannette and Fukami say: “For example, bacterial inoculation more strongly affected sugar composition and the presence of floral nectar than did yeast inoculation, similar to ex situ studies using M. aurantiacus nectar. On the other hand, yeasts decreased the concentration of specific amino acids, while bacteria tended to increase amino acid concentration overall.”

They add: “Microbe-driven divergence in nectar traits may introduce variation in floral attractiveness to particular floral visitors. For example, bumble-bees are attracted to yeast-colonized flowers, while bacterial growth can reduce visitation by diverse floral visitors. These scenarios remain speculative, but suggest that it might be difficult to explain the abundance and distribution of plants, pollinators, or nectar microbes if interactions among the three groups were not considered simultaneously.”